For the second time in two days, the Drainage Services Department was forced to close the Kai Tak Tunnel yesterday when water started leaking.

All lanes were closed at 8:36am but two were reopened half an hour later.

The tunnel was back in full operations by last night.

The department said it is still investigating the source of the leak, but suspects drains at the junction of Kowloon City Road and Mo Cheong Street, which were recently repaired.

During the amber rainstorm warning signal on Friday, water leaked into the tunnel, forcing the closure of all lanes for almost 50 minutes.

Some cars stalled in the tunnel, with a few drivers reportedly seeing "waterfalls."

Kai Tak Tunnel, formerly known as the airport tunnel, connects the Kowloon Bay and To Kwa Wan areas by passing beneath the site of the former airport at Kai Tak.

Completed in 1982, it was then called the Kowloon Bay tunnel and was the first toll-free tunnel in Hong Kong.

A department spokesman said a report was received on April 5 and a preliminary investigation suggested that a stormwater drain, with a diameter of just over a meter, at Mok Cheong Street, which crossed the top of the tunnel, had leaked.

It asked a contractor to repair the leakage, and work is expected to be completed by Wednesday.

Ray Su Kai-leung, associate professor of structural engineering at the University of Hong Kong, said leakages in a tunnel are a big problem.

"It can be a threat to the tunnel's structural integrity if lots of soil is washed away," Su said.

However, there is no evidence yet to suggest this has happened.

"It may be a leaky water pipe. There may also be risks that cars and people could be trapped inside the tunnel," he said.